1. Field of the Invention
The present invention relates to a one-way clutch capable of transmitting the rotation of an input rotational member to an output rotational member, but preventing the rotation of the output rotational member from being transmitted to the input rotational member. Further, it relates to an electric parking brake device using the one-way clutch.
2. Discussion of the Related Art
As electric parking brake devices, there has been known one which is described in Japanese unexamined, published patent application No. 2004-161192. The device is composed of an electric motor having an output shaft rotatable in either of positive and negative-going directions, a rotation transmission mechanism for bringing a parking brake into the braking state when rotationally driven by the electric motor in the positive-going direction but into the braking release state when rotationally driven in the negative-going direction, and a one-way clutch (brake mechanism) provided at a part of the rotation drive train of the rotation transmission mechanism for transmitting the rotation of an input rotational member on the electric motor side to an output rotational member on the parking brake side, but preventing the rotation of the output rotational member from being transmitted to the input rotational member. The one-way clutch is composed of an outer ring having a cylindrical internal surface and locked on a housing, an output shaft rotatably provided in the outer ring and having a sector-shape opening portion formed at one end thereof, an input shaft rotatably provided in the outer ring and having a sector-shape expansion portion which radially protrudes and is housed in the sector-shape opening portion of the output shaft, and a coil spring being in friction contact with the internal surface of the outer ring under a predetermined friction engaging force and having respective radially inwardly curved ends interposed respectively between clearances which are defined between the sector-shape opening portion and the sector-shape expansion portion in the circumferential direction.
In the one-way clutch disclosed in the aforementioned patent application, when the input shaft is given rotation from the electric motor, the sector-shape expansion portion is brought into contact with one end of the coil spring to exert the rotational power on the coil spring, and the rotational power acts to contract the outer diameter of the coil spring regardless of whether the rotational direction is the positive or negative-going direction. Thus, the coil spring slides on the internal surface of the outer ring to rotate together with the input shaft, whereby the output shaft is also rotated as a result of the sector-shape opening portion being pushed at one inner end surface thereof. Conversely, when rotation is given from the parking brake side to the output shaft, one inner end surface of the sector-shape opening portion is brought into contact with one end portion of the coil spring to exert the rotational power on the coil spring, in which case the rotational power acts to expand the outer diameter of the coil spring regardless of whether the direction of the rotational power is the positive or negative-going direction. Thus, because the coil spring is pressured strongly on the internal surface of the outer ring, the sliding of the coil spring on the internal surface of the outer ring is prevented thereby to cause the input shaft not to rotate. That is, rotation is transmitted from the input shaft side toward the output shaft side, but is prevented from the output shaft side toward the input shaft side.
In the aforementioned patent application, as a modified embodiment of the one-way clutch, there is also described one in which a coil spring is engaged at its internal surface portion with the external surface of a shaft locked to the housing, under a predetermined friction engaging force.
As described above, in the one-way clutch described in the aforementioned patent application, it is possible to prevent rotation transmission from the output shaft side toward the input shaft side. However, it may be the case that rotation is given from the output shaft side in dependence on the operation condition that lubrication oil is flown to between the coil spring and the outer ring or that vibration or impact is added to the one-way clutch. In that case, it is likely that the coil spring slides relative to the outer ring and hence, that the operation to prevent rotation transmission toward the input shaft side become unreliable to degrade the function of the one-way clutch.